In conventional run flat tires, a pair of rubber layers having an approximately crescent-shaped cross section for reinforcing side portions (occasionally referred to as side reinforcing rubber layers, hereinafter) are disposed to increase rigidity of the side wall portions. However, when a tire is driven under a decreased inner pressure of a tire due to puncture or the like cause, i.e., in the condition of run flat driving, deformation of the side reinforcing rubber is increased as deformation of the side wall portions of the tire is increased, and heat buildup is increased. The temperature of the tire occasionally reaches 200° C. or higher. Under such a condition, there is the possibility that the condition in the side reinforcing rubber layer exceeds the limit of failure and the tire is damaged.
To improve the performance under the run flat condition, application of styrene-butadiene rubber (SBR) having a high vinyl structure and application of a phenol resin are conducted so that heat resistance of the rubber composition for the side reinforcement is improved.
Suppressing heat buildup of the entire tire is another effective method for improving the performance under the run flat condition. Means for decreasing the heat buildup of the side reinforcing rubber and the ply coating rubber are conducted.
Although these means can exhibit the effect of improving the performance under the run flat condition, the necessity in the market for farther improvement in the performance under the run flat condition is increasing, and a tire exhibiting more excellent performance under the run flat condition is desired.
When a tire damaged after the run flat driving is observed, it is found that cracks first grow in the side reinforcing rubber layer of the run flat tire in the vicinity of the portion having the maximum gauge (thickness) from the side of the inner liner to the side of the ply coating rubber. Separation takes place at the interface of the side reinforcing rubber and the ply coating rubber to which cracks grow, and the side reinforcing rubber is peeled off to damage the tire.
In Patent Reference 1, a technology in which a rubber sheet is disposed extending along the interface of a side reinforcing rubber layer comprising a hydrogenated NBR composition and a ply coating rubber layer comprising a general purpose diene-based rubber so that problems on adhesion of rubber layers due to insufficient compatibility between polymers are overcome, is disclosed.
However, in the technology disclosed in Patent Reference 1, a specific rubber is required, and problems arise from the standpoint of the cost and the control of the material. Therefore, it is desired that the above problems are overcome by using a general purpose rubber and a simpler method.    [Patent Reference 1] Japanese Patent Application Laid-Open No. 2002-30187